Legislation and statutory regulations require self-monitoring capabilities, for example onboard diagnosis, to monitor motor vehicle emissions of hydrocarbons, carbon monoxide, nitrogen oxide, particulates, and the like. In addition, MY15 medium-duty California Air Resources Board (CARB) onboard diagnostics (OBD) regulatory requirements require a worst-case diesel oxidation catalyst (DOC) threshold event in tailpipe emissions to fall within regulatory tailpipe limits. As such, a robust diagnostic method to diagnose selective catalytic reduction (SCR) catalyst feed gas quality in order to detect a failed DOC before tailpipe NOx emissions exceed regulatory limits would be desirable.